Chemical production is a primary consumer of crude oil. Traditionally, straight run naphtha (naphtha being a mixture of hydrocarbons having boiling points less than 200 degrees Celsius (deg C.)) can be used for steam cracking to produce ethylene and propylene, because straight run naphtha contains a greater hydrogen content relative to other feedstocks. In addition, straight run naphtha typically produces limited amounts of hydrocarbons containing more than 10 carbon atoms, also called pyrolysis fuel oil, on the order of 3 weight percent (wt %) to 6 wt % of the total product. Heavier feedstocks, such as vacuum gas oil, can be processed in a fluid catalytic cracking (FCC) unit to produce propylene and ethylene. While an FCC unit can result in the production of high octane-rating gasoline blend stock, it is limited in conversion of feedstock into ethylene and propylene.
Other feedstocks, such as gas oil with a boiling point of greater than 200 deg C., can be used in steam cracking processes, but can result in a lower yield of ethylene and propylene, as well as an increased coking rate due to the heavy molecules in the gas oil fraction. Thus, gas oil fractions do not make suitable feeds for steam cracking processes.
Expanding feedstocks for steam cracking processes to include whole range crude oil or residue fractions is problematic because of the presence of large molecules such as asphaltene in the feedstock. Heavy molecules, particularly, polyaromatic compounds, tend to form coke in the pyrolysis tube and cause fouling in the transfer line exchanger (TLE). A coke layer in the pyrolysis tube can inhibit heat transfer and can cause physical failure of the pyrolysis tube. Severe coking can shorten the run time of the steam cracker, which is one of the most critical parameters in managing the economics of a steam cracker. As a result, the advantage of using cheaper feedstocks, crude oil and heavy residue streams, can be depleted by a short run length of the steam cracking plant. It should be noted that when starting with whole range crude oil or residue fractions the amount of pyrolysis fuel oil can be between 20 wt % and 30 wt % of the total product stream.
Gas oil fractions can be pre-treated in one or more pre-treatment approaches, such as hydrotreatment processes, thermal conversion processes, extraction processes, and distillation processes. Thermal conversion processes can include coking processes and visbreaking processes. Extraction processes can include solvent deasphalting processes. Distillation processes can include atmospheric distillation or vacuum distillation processes. The pre-treatment approaches can decrease the heavy residue fractions, such as the atmospheric residue fraction and the vacuum residue fractions. Thus, decreasing the heavy residue fractions in the feed to the steam cracking feedstock can improve the efficiency of the steam cracking feedstock.
These pre-treatment approaches can process the whole range crude oil before introducing the pre-treated process to the steam cracking process. The pre-treatment approaches can increase light olefin yield and reduce coking in a steam cracking processes. The pre-treatment approaches can increase the hydrogen content of the steam cracking feed—hydrogen content is related to light olefin yield such that the greater the hydrogen content the greater the light olefin yield.
The pre-treatment approaches can decrease the content of heteroatoms, such as sulfur and metals. Sulfur compounds can suppress carbon monoxide formation in a steam cracking process by passivating an inner surface of the pyrolysis tubes. In one approach, 20 wt ppm dimethyl sulfide can be added to a sulfur-free feedstock. However, sulfur content greater than 400 wt ppm in the feedstock to a steam crack process can increase the coking rate in the pyrolysis tubes.
While the pre-treatment approaches can increase the efficiency of a steam cracking process, the pre-treatment approaches also have several drawbacks. First, a hydrotreating process can require a large capital investment and does not remove all undesired compounds, such as asphaltenes. Second, the use of a pre-treatment approach, such as coking, extraction, and distillation, can result in low liquid yield for the feed to the steam cracking process because an amount of the feed is rejected as residue. Third, pre-treatment approaches can require extensive maintenance due to deactivation of catalyst caused by coking, asphaltene deposition, catalyst poisoning, fouling, and sintering of the active species. Finally, many of the pre-treatment processes reject the heaviest fractions of the streams, which reduces overall yield of light olefins and impacts a parameter influence economics of the steam cracker.